Anti-skid control device for vehicle braking apparatus

ABSTRACT

An anti-skid control device comprising a detector for detecting a wheel circumferential velocity, an electromagnetic valve for controlling a hydraulic braking pressure for a wheel and an electronic control circuit for preparing an aimed wheel circumferential velocity based on a detection signal from the detector during a time of braking and controlling the electromagnetic valve so as to bring the wheel circumferential velocity closer to said aimed wheel circumferential velocity.

This invention concerns an anti-skid control device for use in vehiclebraking apparatus.

It has been known that the slip ratio S is defined as: ##EQU1## where Vrepresents a vehicle velocity and Vw represents wheel circumferentialvelocity, as well as that there is a relationship between the slip ratioS and the friction coefficient μ approximately as shown in FIG. 1, inwhich μ represents a friction coefficient between wheels and a roadsurface in contact therewith along the running direction. In FIG. 1, thevalues for μ and μ_(L) taken on the ordinate are normalized referring tothe maximum value as 1. In FIG. 1, curve a represents a relationshipbetween the slip ratio S and the friction coefficient μ at a roadsurface of a high friction coefficient such as a dry concrete roadsurface along the running direction of a vehicle, curve b shows arelationship between the slip ratio S and the friction coefficient μ ata road surface of a low friction coefficient liable to cause slippingsuch as a snow-covered road surface along the running direction of avehicle and curve c represents a relationship between the slip ratio Sand the friction coefficient μ_(L) in the lateral direction of thewheels, that is, in the direction perpendicular to the running directionthereof. As is apparent from FIG. 1, the friction coefficient μ isgenerally maximized near the value of slip ratio S=0.2, and is decreasedas the slip ratio S increases, in a case where the slip ratio is morethan 0.2, for example, in a case where a rapid braking is applied to thevehicle wheels so as to stop rotation of the wheels and lock the wheels,whereas the lateral friction coefficient μ_(L) is abruptly decreasedalong with the increase in the slip ratio S. Accordingly, in order tosupply the braking force to the wheels so as to stop the vehicle withinthe least distance, it is preferred that braking force is applied to thewheels such that the friction coefficient μ may always be kept near themaximum value, that is, the slip ratio S is maintained near 0.2 duringbraking. Further, in the case where the braking is applied such that thefriction coefficient μ near the maximum value may be obtained asmentioned above, since the lateral friction coefficient μ_(L) also takesa relatively high value, the lateral slipping of the wheels is reducedand the vehicle can be stopped with high safety. Accordingly, it hasbeen demanded for such an anti-skid control device for use in vehiclebraking apparatus as capable of controlling the braking force applied tothe wheels such that the slip ratio S may take a preferred value asdescribed above.

By the way, since it is generally difficult to directly detect the slipratio S of a vehicle, for example, automobile during running, there havebeen proposed such anti-skid control devices as capable of satisfyingthe foregoing requirement by using measurable values concerned with theslip ratio S. In one of proposed anti-skid control devices using asimulated vehicle velocity, the vehicle velocity is forecasted by apredetermined simulated vehicle velocity after the start of the brakingoperation and the circumferential velocity of the wheels is controlledbased on the simulated vehicle velocity so as to attain a preferred slipratio.

However, in the anti-skid control device adapted to merely forecast thesimulated vehicle velocity, thereby set an aimed circumferentialvelocity of wheels in view of the slip ratio, and control the supply ofthe braking forces to the wheels so as to conform the actualcircumferential velocity of the wheels to the aimed circumferentialvelocity of the wheels, if a simulated vehicle velocity with a highdeceleration is set for running on a road surface of a low frictioncoefficient, the circumferential velocity of the wheels is abruptlydecreased to bring the wheels into a locked state and increase thebraking distance and tend to cause lateral slip of the wheels, to resultin an instable driving state. On the other hand, if a simulated vehiclevelocity with a low deceleration is set for running on a road surface ofa high friction coefficient, substantially non-braking state isconversely caused to increase the braking distance extremely.

In order to overcome the foregoing disadvantages, there has also beenproposed such an anti-skid control device as measuring the extent of thetime for moderating the braking force in the control for applying thebraking force to the wheels, estimating that the vehicle runs on a roadsurface of a high friction coefficient if the moderating time is shortand, while on the other hand, that the vehicle runs on a road surface oflow friction coefficient if the moderating time is long, and increasingor decreasing the simulated vehicle velocity based on the thus estimatedresult. However, since the moderating time as described above isconcerned with the friction coefficient, as well as varies also inconnection with the inertia moment, braking moment, etc. of the wheelsand the rotating parts related to the wheels, it is difficult to obtaina brake control with a friction coefficient near the maximum value onlywith the increase/decrease of the simulated vehicle velocity dependingon the moderating time.

This invention has been achieved in view of the foregoing problems andit is an object thereof to provide an anti-skid control device capableof applying a braking force to a vehicle corresponding to the change ofthe friction coefficient between wheels and a road surface in contacttherewith, thereby enabling to prevent the wheels effectively fromlocking and shorten the braking distance as much as possible.

According to this invention, the foregoing object can be attained by ananti-skid control device comprising a means for detecting thecircumferential velocity of wheels, an electromagnetic valve means forcontrolling the hydraulic braking pressure for the wheels, and anelectronic control circuit means for preparing an aimed circumferentialvelocity of wheels during a time of braking based on detected signalsfrom the detecting means and controlling the electromagnetic valve meansso as to bring the circumferential velocity of the wheels nearer to theaimed circumferential velocity of the wheels, the electronic controlcircuit means being adapted to determine a ratio of circumferentialacceleration and deceleration of the wheels on every anti-skidcontrolling period for decreasing/increasing the circumferentialvelocity of the wheels, estimate the friction coefficient on a roadsurface from the ratio and control the valve means based on theestimated friction coefficient at the road surface.

These and other objects, features as well as advantageous effects ofthis invention will now be described more in detail referring to thepreferred embodiment shown in the appended drawings, wherein

FIG. 1 is a relationship between the slip ratio and the frictioncoefficient;

FIG. 2 is a block diagram for a preferred embodiment according to thisinvention;

FIG. 3 is a timing diagram of a preferred embodiment shown in FIG. 2;

FIG. 4 is a relationship between the ratio α and the frictioncoefficient μ;

FIGS. 5 and 6 are timing diagrams for explaining operation of theembodiment shown in FIG. 2; and

FIG. 7 is a flow chart of the program in the case of using amicrocomputer instead of the embodiment shown in FIG. 2.

In FIG. 2 and FIG. 3, the pressing force on a brake pedal 1 istransmitted to a master cylinder 2, which generates a hydraulic pressurein accordance with the pressing force and the hydraulic pressure istransmitted to a wheel cylinder 4.

The wheel cylinder 4 generates a braking force f_(b) corresponding tothe thus supplied hydraulic pressure P, and a wheel 5 of an automobileis braked due to the difference between the driving force f_(r) from aroad surface 6 based on the friction coefficient μ relative to the roadsurface 6 in contact with the wheel 5 and the braking force f_(b), bywhich the circumferential velocity Vw of the wheel 5 is determined. Theslip ratio S is determined due to the difference between thecircumferential velocity Vw and the velocity V of a vehicle body 7, thefriction coefficient μ between the wheel 5 and road surface 6 isdetermined depending on the slip ratio S and the velocity V of thevehicle body 7 is varied with the friction force applied from the roadsurface 6 to the wheel 5 based on the thus determined frictioncoefficient μ. A detector 9 comprises for example, an electromagnetic oroptical pulse generator, a counter, a coefficient (wheel diameter)multiplier, etc. for detecting the wheel circumferential velocity Vwfrom the rotating velocity of the wheel 5. The wheel circumferentialvelocity Vw obtained from the detector 9 is supplied to a differentiator10 and a comparator 11. The differentiator 10 differentiates the wheelcircumferential velocity Vw to supply a wheel acceleration/decelerationVw to an acceleration/deceleration ratio calculator 12 and an aimedwheel circumferential velocity calculator 13. The aimed wheelcircumferential velocity calculator 13 calculates the aimed wheelcircumferential velocity Vs from the simulated vehicle velocity Vr basedon the friction coefficient μ supplied from a converter 14 and apreviously determined optimal slip ratio Sr and delivers the velocity Vsto the comparator 11. The comparator 11 compares the actual wheelcircumferential velocity Vw with the aimed wheel circumferentialvelocity Vs and delivers the result of the comparison to a moderatingsignal generator 15. Upon receiving the comparison result that theactual wheel circumferential velocity Vw is lower than the aimed wheelcircumferential velocity Vs from the comparator 11, the generator 15supplies a moderating signal m to the calculator 12 and theelectromagnetic valve 17. 0n the other hand, when the result ofcomparison that the actual wheel circumferential velocity Vw is higherthan the aimed wheel circumferential velocity Vs is received from thecomparator 11, the generator 15 supplies a moderation release signal mto the calculator 12 and the electromagnetic valve 17. The calculator 12stores the acceleration/deceleration Vw from the differentiator 10 asthe deceleration Vwd i.e. the decreasing rate Vwd of the wheelcircumferential velocity Vw by the moderating signal m generated fromthe generator 15 at the time when the actual wheel circumferentialvelocity Vw goes lower than the aimed wheel circumferential velocity Vs.Then, at the time when the actual wheel circumferential velocity Vw goeshigher than the aimed wheel circumferential velocity Vs, the calculator12 reads the acceleration/ deceleration Vw from the differentiator 10 asthe acceleration Vwu i.e. the increasing rate Vwu of the wheelcircumferential velocity Vw by the moderation release signal m generatedfrom the generator 15, as well as calculates the ratio α of theacceleration Vwu to the previously stored deceleration Vwd, that is,

    α=|Vwu/Vwd|

and supplies the thus obtained ratio α to the converter 14. In thiscase, the ratio α has a concern with the friction coefficient μ of theroad surface 6 irrespective of the moment of inertia of the wheel 5. Forexample, the ratio α is increased if the friction coefficient μ ishigher and, conversely, the ratio α is decreased if the frictioncoefficient μ is lower. The relationship between the ratio α and thefriction coefficient μ can be determined from the equation of motion ofa modelled rotating wheel system 16 at the time when braking is appliedthereon and also from the experiment by an actual running of vehicle,which is shown as curve d in FIG. 4. The relationship between the ratioα and the friction coefficient shown by the curve d thus determined ispreviously set or stored to the converter 14 and, accordingly, theconverter 14 supplies a friction coefficient μ corresponding to thesupplied ratio α to the calculator 13.

The electromagnetic valve 17 that receives the moderating signal m andthe moderation release signal m from the generator 15 is actuated suchthat, upon receiving the moderating m, the hydraulic pressure in themaster cylinder 2 is released to the tank of a hydraulic pressure source18 and, while upon receiving the moderation release signal m, thehydraulic pressure from the hydraulic pressure source 18 is supplied tothe wheel cylinder 4 to recover the hydraulic pressure P once moderated.

The operation of the anti-skid control device 30 constituted asdescribed above is as follows. At first, Vro and Sro are previously setto the calculator 13 as the initial values for the deceleration Vr forthe simulated vehicle velocity Vr and the slip ratio Sr. As the initialvalues, values for the road surface of a high friction coefficient areset for the safety, for example, Vro=-1G (where G means gravitationalacceleration) and Sro=0.25. In the vehicle body 7 running at the vehiclebody velocity Vo, when the pressing force is applied on the pedal 1 tosupply a hydraulic pressure from the master cylinder 2 to the wheelcylinder 4 at a time t₀, the calculator 13 starts the timer operation bythe signal from a switch (not illustrated) for detecting the applicationof the pressing force onto the pedal 1 and compares the initial valueVro with the value Vw from the differentiator 10. When the relationshipVw<Vro is detected at a time t₁, the calculator 13 starts to calculatethe simulated vehicle velocity Vr and the aimed wheel circumferentialvelocity Vs while assuming that the wheel 5 starts slipping. That is,the calculator 13 executes the calculation:

    Vs=(Vo+Vro(t-T.sub.0)) (1-Sro)

where t represents the time elapsed after the time t₀, T₀ represents theprogress of time from the time t₀ to t₁, Vro corresponds to the wheeldeceleration Vwl at the time t₁ and (Vo+Vro(t-T₀)) represents theinitial simulated vehicle velocity Vrl. The calculator 13 supplies theresult of the calculation as the first aimed wheel circumferentialvelocity Vsl to the comparator 11. The comparator 11 compares the aimedwheel circumferential velocity Vsl supplied with the wheelcircumferential velocity Vw at present and supplies the result of thecomparison to the signal generator 15. When the relation Vw<Vs1 isreached at the time t₂, the signal generator 15 delivers the moderatingsignal m to the calculator 12 and the electromagnetic valve 17 and, inturn, the electromagnetic valve 17 releases the hydraulic pressure fromthe master cylinder 2 to the tank of the hydraulic pressure source 18 toresult in the decrease of the hydraulic pressure P supplied from themaster cylinder 2 to the wheel cylinder 4. On the other hand, thecalculator 12, upon receiving the moderating signal m, stores the valueVw supplied from the differentiator 10 at the time, that is, at the timet₂ as the deceleration or the decreasing rate Vwd. The wheelcircumferential velocity Vw does not increase simultaneously with thedecreasing of the hydraulic pressure P supplied to the wheel cylinder 4at the time t₂ due to the moment of inertia or the like of the wheel 5.Therefore, the velocity Vw increases after having decreased oncesubsequent to the time t₂. When the relationship Vw>Vsl is reached atthe time t₃, the signal generator 15 delivers the moderation rleasesignal m to the calculator 12 and the electromagnetic valve 17 and thehydraulic pressure in turn, is supplied through the valve 17 from thehydraulic pressure source 18 to the wheel cylinder 4 to recover thehydraulic pressure P decreased previously. On the other hand, uponreceiving the moderation release signal m, the calculator 12 reads thevalue Vw supplied from the differentiator 10 at that time, that is, atthe time t₃ as the acceleration or the increasing rate Vwu and,simultaneously, calculates the ratio α from the decreasing rate Vwdstored at the time t₂ and the increasing rate Vwu as described above andsupplies the result of the calculation to the converter 14. Theconverter 14 determines a friction coefficient μ corresponding to theratio α based on the ratio α thus supplied and supplies the frictioncoefficient μ to the calculator 13. In other words, the converter 14converts the value of ratio α into the value of the friction coefficientμ based on the converting table shown in FIG. 4 which is stored in theconverter 14. The calculator 13 corrects the simulated wheelcircumferential deceleration Vr from Vro to -μG (G: gravitationalacceleration) by the friction coefficient μ thus supplied and comparesthe thus corrected value Vr2=-μG with the value Vw issued from thedifferentiator 10. When the relationsnip Vw<Vr2 is detected at the timet₄, the calculator 13 regards that the wheel 5 starts slipping again andbegins to execute the calculation newly for the simulated vehiclevelocity Vr2 and the aimed wheel circumferential velocity Vs2. That is,the calculator 13 executes the calculation:

    Vs=(Vo+Vr2(t-T.sub.0)) (1-Sro)

in the same manner as described above and delivers the result of thecalculation as a new aimed wheel circumferential velocity Vs2 to thecomparator 11. The succeeding procedures are the same as described aboveand, when the relationship Vw<Vs2 is reached at the time t₅, thegenerator 15 delivers the moderating signal m to order the moderatingoperation for braking. Then, if the relationship Vw>Vs2 is reached atthat time t₆, the generator 15 delivers the moderation release signal mto release the moderating operation. The foregoing operations arerepeated successively hereinafter.

By the way, in the braking device 30 constituted as has been describedabove, since the friction coefficient μ is estimated by thedetermination of the ratio α and the simulated vehicle velocity Vr andthe aimed wheel circumferential velocity Vs are corrected thereby, ananti-skid control substantially corresponding to the change in thefriction coefficient μ can be carried out. Namely, in the case ofapplying braking at a road surface of a high friction coefficient μ,since the wheel is generally less slipping, the wheel circumferentialvelocity Vw is slowly lowered during a time of braking, while velocityVw rapidly increases during a time of releasing the braking, that is,moderating operation, for example, as shown in FIG. 5. On the otherhand, in the case of applying braking at a road surface of a lowfriction coefficient μ, since the wheel is generally liable to slip, thewheel circumferential velocity Vw rapidly decreases during a time ofbraking, while velocity Vw slowly increases during a time of releasingthe braking, for example, as shown in FIG. 6. Accordingly, in aconventional control device in which a fixed simulated vehicle velocityVr and an aimed wheel circumferential velocity Vs are used, if asimulated vehicle velocity Vr with a high deceleration is setirrespective of a road surface of a low friction coefficient μ, forinstance, the wheel circumferential velocity Vw abruptly decreases toinstantly bring about a locked state as shown in FIG. 6 to increase thebraking distance and tend to cause lateral slipping thereby resulting inan instable driving operation. On the other hand, if a simulated vehiclevelocity Vr with a low deceleration is set irrepective of a road surfaceof a high friction coefficient, a substantially non-braking stateresults as shown in FIG. 5, by which the braking distance is remarkablyextended. On the other hand, in the control device according to thisinvention, since the friction coefficient μ is determined from the ratioα and the simulated vehicle velocity is corrected with the frictioncoefficient μ, an appropriate braking can be carried out correspondingto the change in the friction coefficient μ. In addition, since thesubstantial moment of inertia Iw of the wheel is increased several timesas large as only that of the wheels depending on the shifting positionof a transmission gear, for example, top, third or second velocityposition upon braking, the change in the wheel circumferential velocityduring braking operation results depending on the value for the momentof inertia Iw. Since the ratio α is greater if the friction coefficientis high and smaller if it is low irrespective of the extent of themoment of inertia Iw of the wheels, preferred braking characteristicscan also be obtained by the control device according to this inventionfor the increase or decrease of the substantial moment of inertia of thewheel.

Although the electronic control circuit 31 is constituted with thedifferentiator 10, comparator 11, calculator 12, calculator 13,converter 14 and generator 15 in the embodiment described above, it mayalternatively be possible to constitute the electronic control circuit31 with a microcomputer or the like and the same operation as describedabove can be obtained by operating the microcomputer on a programcomprising steps as shown in FIG. 7. The program is started by a signalfrom a switch actuated by the pressing of pedal 1, sets the initialvalues Vro and Sro for the simulated vehicle deceleration Vr and theslip ratio S at the step 40, calculates the wheel circumferentialvelocity Vw and the wheel circumferential acceleration/deceleration Vwby the signal from the detector 9 at the step 41, decides whether thewheel acceleration/deceleration Vw is greater than the initial value Vroor not at the step 42, in which the program returns to the step 41 if itis not less than the initial value Vro or proceeds to the step 43 if itis less than the initial value Vro. The program sets, at the step 43,the wheel circumferential velocity Vo at the time of transition from thestep 42 to the step 43 and the time T₀ from the starting step to thestep 43. The program calculates the above-described aimed value:

    Vs=(Vo+Vro(t-T.sub.0)) (1-Sro)

at the step 44, reads in the present wheel circumferential velocity Vwat the step 45, decides whether the wheel circumferential velocity Vwthus read has been lowered or not as compared with the aimed wheelcircumferential velocity Vs and returns to the step 44 if it is notlower than the aimed wheel circumferential velocity Vs or delivers themoderating signal m if it is lowered at the step 47 and calculates thewheel acceleration/deceleration Vw at that time and stores the value asthe decreasing rate Vwd at the step 48. The program then calculates theaimed wheel circumferential velocity Vs again at that time at the step49, reads in the wheel circumferential velocity Vw at that time at thestep 50, decides at the step 51 whether the wheel circumferentialvelocity Vw having been read in at the step 50 is increased or not ascompared with aimed wheel circumferential velocity Vs calculated in thestep 49 and returns to the step 49 if it is not increased and deliversthe moderation release signal m if it is increased at the step 52,calculates the wheel acceleration/deceleration Vw at the time beginningto deliver the moderation release signal m in order to obtain theincreasing rate Vwu at the step 53, calculates the ratio α from thedecreasing rate Vwd having been stored in the step 48 and the increasingrate Vwu obtained in the step 53 at the step 54 and determines afriction coefficient μ corresponding to the ratio α from the thuscalculated ratio α at the step 55. Upon determining the frictioncoefficient μ, the curve d shown in FIG. 4 is previously stored as aconverting table in a memory device. At the step 56, the simulatedvehicle acceleration/deceleration Vr is corrected into -μG by thefriction coefficient μ obtained in the step 55 and, thereafter, theprogram returns to the step 41 and then repeats the foregoing steps.

Although the slip ratio S has been fixed in the above-describedembodiment, this invention is no way limited only thereto but the slipratio Sr may be corrected together with the correction for the simulatedvehicle acceleration/ deceleration Vr in order to avoid the phenomenontending to cause locking at a road surface of a low friction coefficientand a phenomenon tending to result in insufficient braking caused at aroad surface of a high friction coefficient. In this case, as oneexample of corrected set values for the simulated vehicleacceleration/deceleration Vr and the slip ratio Sr relative to thefriction coefficient μ, those values can be experimentally obtained, forexample, for one group of the slip ratio Sr; such as the slip ratioSr=0.3-0.4 for a high friction coefficient, that is, if the frictioncoefficient is greater than 0.6, the slip ratio Sr=0.2-0.3 for a mediumfriction coefficient, that is, if the friction coefficient is between0.35 and 0.6, and the slip ratio Sr=0.1-0.2 for a low frictioncoefficient, that is, if the friction coefficient is less than 0.35 andfor the other group of the slip ratio Sr: such as the slip ratioSr=0.1-0.3 for the high friction coefficient, the slip ratio Sr=0.1-0.2for the medium friction coefficient and the slip ratio Sr=0.05-0.1

Further, although the curve d is used in the foregoing embodiment in thecase of determining the friction coefficient μ corresponding to a ratioα from this ratio α, a conversion characteristic approximate to astraight or step-wise broken line above the curve d as shown by thecurve e or f may be used for the safety, because the frictioncoefficient μ is estimated smaller in the region 60 below the curve d,where the braking operation is too weak and results in a non-brakingstate. Furthermore, although the description has been made in theforegoing embodiment with respect to a mono-wheel system, this inventionis no way limited only thereto but the foregoing braking may be appliedto each of four wheels of an automobile. In addition, the braking mayalso be applied to the right and left front wheels respectively and theright and left rear wheels respectively. In addition, the braking mayalso be applied to a dual-system in which pipe lines for front and rearwheel brakes are disposed in an X-like configu ration.

As has been described above according to this invention, since theelectronic control circuit is adapted to determine circumferentialacceleration/deceleration ratio of the wheel on every anti-skidcontrolling period of decreasing and increasing wheel circumferentialvelocity, estimate the friction coefficient on the road surface from theacceleration/deceleration ratio and control the electromagnetic valvefor controlling the hydraulic braking pressure of the wheel based on theestimated friction coefficient on the road surface, an extremelydesirable anti-skid control can be carried out even to a remarkablechange in the friction coefficient μ to shorten the braking distance andapply braking while desirably maintaining the stability for the drivingoperation.

What is claimed is:
 1. An anti-skid control device comprising means fordetecting a wheel circumferential velocity, an electromagnetic valvemeans for controlling a hydraulic braking pressure for a wheel, and anelectronic control circuit means for preparing an aimed wheelcircumferential velocity based on a detection signal from said detectingmeans during a time of braking and controlling said electromagneticvalve means so as to bring the wheel circumferential velocity closer tosaid aimed wheel circumferential velocity, said circuit means furtherbeing adapted to determine a ratio of circumferential acceleration anddeceleration of the wheel on every anti-skid controlling period fordecreasing/increasing the wheel circumferential velocity, estimate afriction coefficient on a road surface from the ratio and control saidelectromagnetic valve means based on the thus estimated frictioncoefficient on the road surface.
 2. The anti-skid control deviceaccording to claim 1, wherein said circuit means is adapted toadaptationally correct said aimed wheel circumferential velocity fromsaid estimated friction coefficient on the road surface to therebycontrol the electromagnetic valve means.
 3. The anti-skid control deviceaccording to claim 2, wherein said circuit means is adapted to correct asimulated wheel circumferential deceleration by said estimated frictioncoefficient on the road surface to thereby adaptationally correct saidaimed wheel circumferential velocity.
 4. The anti-skid control deviceaccording to claim 2, wherein said circuit means is adapted to correctthe simulated wheel circumferential deceleration and the slip ratio bysaid estimated friction coefficient on the road surface to therebyadaptationally correct said aimed wheel circumferential velocity.